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Blue-green algae bloom

Cyanobacteria (Blue-green Algae)

CDC Veterinarian Reference

The information below is taken from the CDC Veterinarian Reference and Cyanobacteria Blooms FAQ brochures.

For more information from the CDC go to

What are cyanobacteria?

Cyanobacteria, sometimes called blue-green algae, are microscopic organisms that live in all types of water.

What is a cyanobacteria blooms?

Cyanobacteria grow quickly, or bloom, when the water is warm, slow-moving, and full of nutrients.

What are some characteristics of cyanobacteria blooms?

  • Cyanobacteria usually bloom during the summer and fall. However, they can bloom anytime during the year.
  • When a bloom occurs, scum might form on the water’s surface.
  • Blooms can be many different colors, from green or blue to red or brown.
  • As the bloom dies off, you might smell an odor that is similar to rotting plants.

What is a toxic bloom?

  • Sometimes, cyanobacteria produce toxins.
  • The toxins can be present in the cyanobacteria cells or in the water.

Other important things to know:

  • Swallowing water that has cyanobacteria or cyanobacterial toxins in it can cause serious illness.
  • Dogs might have more severe symptoms than people, including collapse and sudden death after swallowing the contaminated water while swimming or after licking cyanobacteria from their fur.
  • There are no known antidotes to these toxins. Medical care is supportive.

You cannot tell if a bloom is toxic by looking at it.

Report a Cyanobacteria Bloom

To report a cyanobacteria bloom or related health event call your local or state health department (

Exposure and Clinical Information

Information about the health effects from exposure to cyanobacteria and toxins is derived from reports of animal poisonings.*

Potential Exposure Route: Swallowing water that is contaminated with cyanobacteria or toxins or licking it off fur or hair

  • Hepatotoxins and nephrotoxins
    • Likely Symptoms and Signs: 
      • Excess drooling, vomiting, diarrhea, foaming at mouth
      • Jaundice, hepatomegaly
      • Blood in urine or dark urine
      • Malaise
      • Stumbling
      • Loss of appetite
      • Photosensitization in recovering animals
      • Abdominal tenderness
    • Time to Symptom Onset: Minutes to hours
    • Differential Diagnosis: Acetaminophen or NSAID overdose, rodenticide ingestion, aflatoxicosis and other hepatotoxin poisonings
  • Neurotoxins
    • Likely Symptoms and Signs: 
      • Progression of muscle twitches
      • For saxitoxin, high doses may lead to respiratory paralysis and death if artificial ventilation is not provided.
    • Time to Symptom Onset: Minutes to hours
    • Differential Diagnosis: Pesticide poisoning, myasthenia gravis, other toxin poisoning
    • Possible Laboratory or Other Findings: Presence of toxin in clinical specimens from stomach contents taken from animals that became ill

Potential Exposure Route: Skin contact with water contaminated with cyanobacteria or toxin(s)

  • Dermal toxins
    • Likely Symptoms and Signs: 
      • Rash, hives, allergic reaction
    • Time to Symptom Onset: Minutes to hours
    • Differential Diagnosis: Other dermal allergens
    • Possible Laboratory or Other Findings: Blue-green staining of fur or hair


  1. Monogastric animals appear less sensitive than ruminants or birds; however, the dose-response curve is very steep in dogs—up to 90% of a lethal dose may elicit no clinical signs.
  2. There are no known antidotes to these toxins. Medical care is supportive. Activated charcoal may be useful within the first hour, and atropine has efficacy with saxitoxin exposure.

*References are available at:

Cyanobacteria Blooms FAQs

What are cyanobacteria?

Cyanobacteria, also called blue-green algae, are microscopic organisms found naturally in all types of water. These single-celled organisms live in fresh, brackish (combined salt and fresh water), and marine water. These organisms use sunlight to make their own food. In warm, nutrient-rich (high in phosphorus and nitrogen) environments, cyanobacteria can multiply quickly, creating blooms that spread across the water’s surface. The blooms might become visible.

How are cyanobacteria blooms formed?

Cyanobacteria blooms form when cyanobacteria, which are normally found in the water, start to multiply very quickly. Blooms can form in warm, slow-moving waters that are rich in nutrients from sources such as fertilizer runoff or septic tank overflows. Cyanobacteria blooms need nutrients to survive. The blooms can form at any time, but most often form in late summer or early fall.

What does a cyanobacteria bloom look like?

You might or might not be able to see cyanobacteria blooms. They sometimes stay below the water’s surface, they sometimes float to the surface. Some cyanobacteria blooms can look like foam, scum, or mats, particularly when the wind blows them toward a shoreline. The blooms can be blue, bright green, brown, or red. Blooms sometimes look like paint floating on the water’s surface. As cyanobacteria in a bloom die, the water may smell bad, similar to rotting plants.

Why are some cyanbacteria blooms harmful?

Cyanobacteria blooms that harm people, animals, or the environment are called cyanobacteria harmful algal blooms. Harmful cyanobacteria blooms may affect people, animals, or the environment by:

  • Blocking the sunlight that other organisms need to live. Cyanobacteria blooms can steal the oxygen and nutrients other organisms need to live.
  • Making toxins, called cyanotoxins. Cyanotoxins are among the most powerful natural poisons known. They can make people, their pets, and other animals sick. Unfortunately, there are no remedies to counteract the effects.
  • You cannot tell if a bloom has toxins by looking at it.

How can people and animals come in contact with cyanobacteria and cyanotoxins in the environment?

People and animals can come in contact with cyanobacteria and cyanotoxins that are in the environment by:

  • Drinking water that comes from a lake or reservoir that has a cyanobacteria bloom.
  • Swimming or doing other recreational activities in or on waters that have cyanobacteria blooms.

How do I protect myself, my family, and my pets from cyanobacteria blooms?

To protect yourself, your family and your pets from cyanobacteria blooms:

  • Don’t swim, water ski, or boat in areas where the water is discolored or where you see foam, scum, or mats of algae on the water’s surface.
  • Do not allow children or pets to play in or drink scummy water.
  • If you do swim in water that might contain harmful cyanobacteria, rinse off with fresh water as soon as possible afterward.
  • Don’t let pets or livestock swim in or drink from areas where the water is discolored or where you see foam, scum, or mats of cyanobacteria on the water’s surface.
  • If pets, especially dogs, swim in scummy water, rinse them off immediately. Do not let them lick the cyanobacteria off their fur.
  • Report any “musty” smell or taste in your drinking water to your local water utility.
  • Follow any water-body closures announced by local public health authorities.

Why do dogs get sick more often than people from cyanobacteria blooms?

Dogs will get in a body of water even if it looks or smells bad, including when it contains cyanobacteria. Dogs are also more likely to drink the contaminated water.

How are people or animals that have been exposed to cyanobacteria toxins treated?

If you or your pet comes in contact with a cyanobacteria, wash yourself and your pet thoroughly with fresh water.

  • If you or your pet swallow water from where there is a harmful algae bloom, call your doctor, a Poison Center, or a veterinarian.
  • Call a veterinarian if your animal shows any of the following symptoms of cyanobacteria poisoning: loss of appetite, loss of energy, vomiting, stumbling and falling, foaming at the mouth, diarrhea, convulsions, excessive drooling, tremors and seizures, or any other unexplained sickness after being in contact with water.

How can you help reduce cyanobacteria blooms from forming?

To help reduce cyanobacteria from forming:

  • Use only the recommended amounts of fertilizers on your yard and gardens to reduce the amount that runs off into the environment.
  • Properly maintain your household septic system.
  • Maintain a buffer of natural vegetation around ponds and lakes to filter incoming water.

Is there testing for cyanobacteria toxins?

Yes, but the testing is specialized and can only be done by a few laboratories. Scientists are working to develop toxin test kits for water resource managers and others.

What is CDC doing to address concerns about cyanobacteria blooms?

The Centers for Disease Control and Prevention (CDC) is working to understand and prevent the health effects associated with cyanobacteria blooms by conducting surveillance on human and animal illnesses that are associated with exposures to cyanobacteria blooms in recreational and drinking waters.